It is well known that internal combustion engines operate most efficiently within a limited range of engine revolutions-per-minute (rpm). An engine with limited rpm range will require the use of many gears to enable a vehicle powered by the engine to be used over its desired speed range, which may be from standstill to the national speed limit or some other upper limit. A conventionally operated gearbox requires the vehicle driver to continually shift gears to maintain engine rpm within the optimum range. There are also available transmissions which provide a continuously variable input/output rotational speed ratio, but these are expensive to manufacture and inefficient to operate.
A conventional six-speed gearbox configuration is shown in FIG. 1 as comprising an input shaft 110 for receiving input torque. A first input gear 112 and a second input gear 116 are mounted coaxially on input shaft 110 for rotation relative to the input shaft. First input gear 112 is meshed with a first output gear 114 to form a first gear pair having a first gear ratio determined by the respective diameters or numbers of teeth of the first input gear and the first output gear. Likewise, second input gear 116 is meshed with a second output gear 118 to form a second gear pair having a second gear ratio. The first and second output gears 114 and 118 are mounted coaxially on an output shaft 140 and are fixed to output shaft 140 to rotate together therewith. A third input gear 122 and a fourth input gear 126 are mounted coaxially on input shaft 110 for rotation relative to the input shaft. Third input gear 122 is meshed with a third output gear 124 to form a third gear pair having a third gear ratio. In similar fashion, fourth input gear 126 is meshed with a fourth output gear 128 to form a fourth gear pair having a fourth gear ratio. The third and fourth output gears 124 and 128 are mounted coaxially on output shaft 140 and are fixed to the output shaft to rotate together therewith. A fifth input gear 132 and a sixth input gear 136 are also mounted coaxially on input shaft 110 so as to permit rotation relative to the input shaft. Fifth input gear 132 is meshed with a fifth output gear 134 to form a fifth gear pair having a fifth gear ratio. Likewise, sixth input gear 136 is meshed with a sixth output gear 138 to form a sixth gear pair having a sixth gear ratio. Like the first through fourth output gears, the fifth and sixth output gears 134 and 138 are mounted coaxially on output shaft 140 and are fixed to the output shaft 140 to rotate together therewith. A first selector 152 rotating with input shaft 110 is arranged movably between first gear pair 112, 114 and second gear pair 116, 118 to select either of the first and second gear pairs, a second selector 154 rotating with input shaft 110 is arranged movably between third gear pair 122, 124 and fourth gear pair 126, 128 to select either of the third and fourth gear pairs, and a third selector 156 rotating with input shaft 110 is arranged movably between fifth gear pair 132, 134 and sixth gear pair 136, 138 to select either of the fifth and sixth gear pairs.
The selector positions for the six available speeds (gears), not including neutral (already shown in FIG. 1), are illustrated in FIGS. 2A through 2F. First gear is selected by moving the first selector 152 to the left as shown in FIG. 2A to drivably engage first input gear 112, whereby the first gear ratio associated with first gear pair 112, 114 is chosen and is outputted by output shaft 140. Second gear is chosen by moving first selector 152 to the right as shown in FIG. 2B to drivably engage second input gear 116, whereby the second gear ratio associated with second gear pair 116, 118 is chosen and transmitted through output shaft 140. In similar fashion, second selector 154 is moved to the left to select third gear (FIG. 2C) and to the right to select fourth gear (FIG. 2D), and third selector 156 is moved to the left to select fifth gear (FIG. 2E) and to the right to select sixth gear (FIG. 2F). Thus, there are six gear pairs and six selectable speeds. Under a conventional configuration, two additional gear pairs and an additional selector are required to provide eight selectable speeds.
Therefore, a need exists to provide more discreet selectable speeds without adding gears and selectors to the gearbox configuration, as these bring added weight to the gearbox that is of course undesirable from the standpoint of fuel efficiency. Stated differently, a need exists to maximize the number of discreet speeds available using a given number of gear pairs and selectors.